This invention relates to high vacuum sterilizer testing using a test pack. Such testing and a standard test pack therefor are prescribed by the Association for the Advancement of Medical Instrumentation (AAMI) in AAMI ST. 1-1980 entitled Good Hospital Practice: Steam Sterilization and Sterility Assurance. The AAMI test pack is essentially a body of material made up of articles which are the same as, or quite like, articles which are commonly sterilized in high vacuum sterilizers. In particular, o the body of material may be a stack of folded similar towels, perhaps tied in a bundle, variously called a towel pack, or a Bowie-Dick type test pack, which has a heat-sensitive indicator in some inner portion thereof. Typically, this indicator will be a sheet of material sandwiched in the towels and having the same area as one of the towels as folded. The sheet of material will be covered by a chemical which undergoes a lasting visual change at every point thereof attaining a given temperature.
In operation, the test pack is placed in the sterilizer and subjected to a high vacuum in order to remove air from the towels which, of course, are porous and so tend to be saturated with any ambient fluid medium. Freed from air, the pack is next subjected to steam which is supposed both to penetrate the interstitial space of the towel material making up the test pack and to transfer sterilizing heat to the fibers of that material.
As in known, particles or bubbles or pockets of air contacting portions of material of the test pack insulate the affected material from the steam. If one supposes that there has been effective removal of air due to the mechanical effect of the high vacuum, then one might also suppose that the heat-sensitive material covering of the indicator would reflect this by exhibiting everywhere visually-detectible change due to steam contacting it everywhere. However, if there had been air contacting any part of the coating during steam application to the test pack, then one would suppose that less or no change would be detectible visually at such part.
Inasmuch as articles, materials, etc., which the sterilizer normally receives for sterilization, normally only differ from a test pack in their response to the vacuum and steam treatment cycle by not showing signs of exposure to heat, one supposes therefore that if a test pack subjected to a treatment cycle indicates that all air has been effectively been removed from it and that the steam has contacted every possible particle thereof, then the sterilizer will be effective both to remove air effectively and sterilize properly the aforesaid articles, materials, etc.
The above-said towel-pack testing is described in detail by AAMI ST. 1-80, pages 8, 9 and 10, and, as well, by J. H. Bowie et al in their article "The Bowie and Dick Autoclave Tape Test", The Lancet, Mar. 16, 1963, pages 586 and 587. Here, the towel pack is described as being reusable, subject to being reconditioned after each use. Inasmuch as it is customary to perform the test once daily, or oftener, workers in the art have also devised inexpensive test packs which nevertheless for test purposes are effectively equivalents of the standard towel packs, and are disposable after a single use, and are, stockable and need little or no preparation for use. In one case, the Bowie et al "29 huckaback towels" were replaced by a number of 9".times.12" sheets of absorbent paper stacked about 3/4" inch high. In the middle of this stack was an indicator consisting of one 9".times.12" inch sheet of paper, one side of which was coated with heat sensitive material in a distinctive pattern covering the entire surface of that side. The pack was completed by a crepe paper wrapping secured by a fastener in the form of a piece of sticky tape having a heat sensitive coating on the side opposite its sticky side.
When this prior art test pack is subjected to the test cycle, the heat sensitive material on the fastener shows whether or not the pack had been exposed to heat. If exposure to heat is indicated, then the crepe paper wrapping will be opened to permit getting at the indicator sheet in the middle of the paper stack and inspecting it to see if the steam had contacted the entire surface of the indicator sheet. Complete exposure, and therefore effective air removal would be indicated if the entire pattern of heat sensitive material had changed its appearance substantially uniformly, e.g., from a light colored pattern barely distinguishable from that of the side of the paper on which the pattern was laid, to a pattern entirely of a black or very dark color contrasting strongly with the paper color. Where, if anywhere, air was present next to the indicator sheet's coated surface, no or little color change would have occurred and this would be immediately noticeable as a light-colored interruptions of the otherwise darkened pattern.
With this test pack, the absorbent paper would be chosen in characteristics (nature, porosity, basis weight, etc.) and quantity, such that the paper stack is equivalent to the towels in respect of air and steam permeability. The crepe paper wrapping, and the paper of the indicator sheet in the stock, themselves make no appreciable contribution to performance of the pack. The central location in the stack of the indicator sheet makes its character irrelevant, whereas the crepe paper, for example, surgical crepe, so-called, is relatively transparent to steam.